For successful implantation and pregnancy the importance of synchrony between the embryonic development and the endometrial differentiation has been demonstrated by egg transfer in various species by many workers. Very little work however has been reported on the successful asynchronized egg transfer and successful egg transfer to the non-ovulated recipients. In order to advance our knowledge on the mechanism of implantation and hormonal requirements for implantation and pregnancy and to improve the efficacy of egg transfer procedures, it is proposed: (1) to transfer rabbit eggs at different stages of development to the asynchronized recipients treated with estrogen in various ways and (2) to transfer rabbit eggs to the non-ovulated recipients treated with various potent and long lasting progestins or with estrogen.